Regulation of nonadiabatic processes in the photolysis of some carbonyl compounds

Abstract

Carbonyl compounds studied are confined to acetyl halide (CH₃COCl), acetyl cyanide (CH₃COCN), acetyl sulfide (CH₃COSH), acetaldehyde (CH₃CHO), and methyl formate (HCOOCH₃). They are asymmetrically substituted, but do not follow the well-known Norrish type I reactions. Each compound ejected in an effusive beam at about 300 K is commonly excited to the ¹(n, π*)_CO lower state; that is, a nonbonding electron on O of the CO group is promoted to the antibonding orbital of π*_CO. The photolysis experiments are conducted in the presence of Ar gas and the corresponding fragments are detected using time-resolved Fourier-transform Infrared (FTIR) emission spectroscopy. The enhancement of the collision-induced internal conversion or intersystem crossing facilitates the dissociation channels via highly vibrational states of the ground singlet (S_o) or triplet (T₁) potential energy surfaces. In this manner, an alternative nonadiabatic channel is likely to open yielding different products, even if the diabatic coupling strength is strong between the excited state and the neighboring state. For instance, the photodissociation of CH₃COCl at 248 nm produces HCl, CO, and CH₂ fragments, in contrast to the supersonic jet experiments showing dominance of the Cl fragment eliminated from the excited state. If the diabatic coupling strength is weak, dissociation proceeds mainly through internal conversion, such as the cases of CH₃COCN and CH₃COSH. The photodissociation of CH₃COCN at 308 nm has never been reported before, while for CH₃COSH matrix-isolated photodissociation was conducted that shows a distinct spectral feature from the current FTIR method. The CH₃CHO and HCOOCH₃ molecules belong to the same type of carbonyl compounds, in which the molecular products, CO + CH₄ and CO + CH₃OH, are produced through both transition state and roaming pathways. Their products are characterized differently between molecular beam and current FTIR experiments. For instance, the photodissociation of HCOOCH₃ at 248 nm yields CO with the vibrational state v ≥ 4, in contrast to the molecular beam experiments producing CO at v = 1. The photodissociation of CH₃CHO at 308 nm intensifies a low energy component in the CH₄ vibrational distribution, thus verifying the transition state pathway for the first time.